1. Field of the Invention
The present invention relates to a developing device, for example, used in a copying machine, a printer, or a facsimile machine of an electro-photographic type, an electronic recording type, or the like, for developing an electrostatic image formed on an image bearing member by supplying developers (toners) from first and second developer bearing members.
2. Description of the Related Art
In recent years, image forming apparatuses such as a copying machine and a printer have been required to output a printed image at increasingly higher speeds.
However, in a known developing device of the image forming apparatus, when a toner supplied from a toner container is held on the surface of one developing roller and an electrostatic latent image is developed (i.e., is visualized) by jumping (i.e., accreting) the toner on the latent image formed on a photoreceptor in an electrostatic manner, the toner held on the surface of the developing roller which is rotating at a high speed because of the speedup requirement quickly deteriorates.
In addition, the speedup causes the developer borne on the developing roller to jump on the photoreceptor and develop the latent image in a short period of time, thereby leading to a loss in image quality.
To solve the above-described problems, a developing device having a plurality of developing rollers for developing a latent image at each roller has been proposed. With this developing device, the deterioration of the toner and the loss in image quality of the high-speed image forming apparatus can be suppressed.
For example, as shown in FIG. 4, a developing device 101 has a structure in which a toner container 102 for containing a toner t has an opening opposing a photosensitive drum 100 where two developing rollers 103a and 103b are disposed. The surfaces of the developing rollers 103a and 103b are coated with thin films of the toner t by two regulating blades 104a and 104b. 
In this developing device 101, the toner t is supplied from a toner supplying container 106 to the toner container 102 via a toner conveying device 107 on the basis of the data of a toner amount detected by a toner-amount detecting sensor 105.
Also, as shown in FIG. 5, the developing device 101 has another structure in which toner containers 102a and 102b have developing rollers 103a and 103b, respectively. In this developing device 101, the toner t is supplied from the toner supplying container 106 to the toner containers 102a and 102b via corresponding toner conveying devices 107a and 107b on the basis of the data of toner amounts detected by corresponding toner-amount detecting sensors 105a and 105b. 
Although the known developing device 101 shown in FIG. 4 having only one toner container 102 and one toner conveying device 107 has a reduced size advantage, when the amount of the toner t in the toner container 102 varies in the range H shown in the figure, a supply amount of the toner t supplied to the developing roller 103a is different from that supplied to the developing roller 103b. 
That is, as shown in FIG. 6, since a toner coating amount coated on the surface of the developing roller 103a disposed upstream with respect to the rotating direction of the photosensitive drum 100 varies depending on the amount of the toner t in the toner container 102, the toner coating amount decreases as a supply amount of the toner t supplied to the developing roller 103a decreases, resulting in a loss in image quality of an output image.
On the other hand, in the developing device 101 shown in FIG. 5, since supply amounts of the toner t supplied to the toner containers 102a and 102b are adjusted by the corresponding toner conveying devices 107a and 107b, stable amounts of the toner t are coated on the surfaces of the developing rollers 103a and 103b. However, since the toner conveying devices 107a and 107b are disposed for supplying the toner t to the corresponding toner containers 102a and 102b, a space for housing two toner conveying devices is required, thereby resulting in an increased size of the developing device 101.
In particular, since the image forming apparatus not only performing at a high speed but also having a reduced size has been required in recent years, a compact developing device having a plurality of developing rollers also has been desired.
It is an object of the present invention to provide a developing device which has a reduced size and stably supplies a developer or a toner.
A developing device comprises a first developer container for containing a developer; a first developer bearing member for bearing and conveying the developer in the first developer container; a second developer container for containing a developer; and a second developer bearing member for bearing and conveying the developer in the second developer container. An electrostatic image formed on an image bearing member is developed by supplying the developers from the first and second developer bearing members to the electrostatic image. The developing device further comprises developer supplying means for supplying the developer in a developer supplying container to the first developer container; and developer conveying means for conveying the developer in the first developer container to the second developer container in order to supply the developer to the second developer container.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.